Neutrino

Neutrino, an elementary particle that is electrically neutral and of very small mass. Neutrinos are created in many types of interaction between elementary particles. Enormous numbers of neutrinos travel through space in cosmic rays. They react so rarely with other particles that they can travel through the whole Earth with only a tiny proportion being absorbed. Trillions pass through every human being in every second, yet we are completely unaware of them.

The neutrino is a fermion—that is, it has a spin of y (in units of h/2p, where h is Planck’s constant). Around 1930 it was observed that in beta-decay (electron-emission) processes the total energy, momentum, and spin were apparently not conserved (see Conservation Laws; Radioactivity). In 1931 the Austrian physicist Wolfgang Pauli suggested that an unobserved particle was being given out in these processes, carrying away some of the energy, momentum, and spin. This particle was later named “neutrino” (Italian for “little neutral one”). Because it has no charge and negligible mass, the neutrino is extremely elusive; however, conclusive proof of its existence was obtained in 1956 by the American physicists Frederick Reines and Clyde Lorrain Cowan, Jr.

The particle emitted in electron beta decay is actually an antineutrino, whereas a neutrino is emitted in positron beta decay. Furthermore, there are two other kinds of neutrino apart from this “electron neutrino”. The muon neutrino is produced, along with a muon, in the decay of a pion. A third type of neutrino, the tau neutrino, also exists (with its antiparticle). It appears in interactions that involve the tau particle. See Standard Model.

Neutrinos can be detected on the very rare occasions that they interact with the nucleus of an atom. One kind of neutrino detector consists of thousands of cubic metres of a liquid very like dry-cleaning fluid in a giant tank in a salt mine. The rock surrounding the tank cuts out other, unwanted kinds of particles in cosmic rays. Neutrinos are detected by the flashes of light given out when they interact with atoms in the liquid. Such “neutrino telescopes” observe neutrinos from the heart of the Sun and from other celestial objects, such as the supernova seen in a nearby galaxy in 1987.

In 2001, measurements from the Sudbury Neutrino Observatory, Ontario, combined with others taken in Japan in 1998, confirmed that neutrinos oscillate—that is, they can rapidly change from one form to another and back again. It was also confirmed that the mass of the neutrino was less than about 10^-7 of the mass of an electron, meaning that the gravitational attraction of all the neutrinos contained in the universe would be too small to prevent it from continuing to expand. The mass of the neutrino would also make it too small to account for the presence of dark matter in the universe. See Future of the Universe.